The flexible structure of the spine is provided by a series of interlocking individual members or vertebrae that are naturally shaped and aligned to provide a gentle undulation which causes the spine to have a gentle S-shaped curve. The vertebrae, each have a drum shaped front, (i.e. anterior portion), the vertebral bodies, collectively aligned to form a column and a rear, (i.e. posterior portion), the pedicles, lamina and spinous processes that interlock at the back of the spine. The vertebral bodies have generally oval-shaped top and bottom plates, (i.e. end plates), of hard cortical bone which surround the softer interior cancellous bone. The spaces between the vertebral bodies are maintained by intervertebral discs which are comprised of a tough outer fibrous material which encases a softer gelatinous mass.
In a healthy spine, the cervical and lumbar areas curve forward, i.e., they are lordotic, while the thoracic region curves in reverse of this. The discs act as spacers to maintain the position of the vertebrae and to transmit and absorb loads between the hard bones of the vertebral bodies.
The intervertebral discs include a taper, or wedge shape which help to define the relative position of the vertebral bodies and to cause the curve of the spinal column. However, it is possible for these discs to lose their shape through traumatic damage or long term degeneration. This condition can result in a loss of the normal spinal curvature or of proper alignment of the vertebrae. As a result, the natural transmission of loads in the spine may be seriously impaired with attendant loss of function and back pain.
An accepted treatment for disc degeneration is to remove the degenerative disk and promote fusion of the remaining adjacent vertebrae. Typically, this procedure incorporates the use of an invertebral body spacer in conjunction with bone graft material that promotes the growth of two vertebrae into a solid segment. The spacer is used to maintain the desired spacing and alignment while the bone graft or bone growth promoting material promotes the fusion of the vertebrae for long term stability. During the fusion process it is desirable to inhibit any relative movement of the vertebrae that are involved.
Various structures have been designed to act as interbody spacers or cages. Implantation of these structures may occur from an anterior, posterior, anterolateral or lateral position. The vertebral spacer or cage of the present invention is designed for anterior insertion, but may also be implanted anterolaterally or laterally. This cage forms a tapered circlet which can be envisioned as a closed bracelet shape. It may have a taper in height from front to back to provide a wedge shape from front to back. It has an opening in its center which receives bone graft material. Side fenestrations, or open areas permit radiographic examination of the fusion process. Further, the peripheral outline of the disc defines generally concentric interior and exterior walls. At the front, or anterior surface, the cage has a substantially solid and smooth or fluid wall (the term “fluid” is used herein to mean that this surface is substantially continuous and devoid of sharp transitions or edges). The anterior wall has screw passages therethrough for countersunk screw heads. Further there are a pair of openings in the anterior face that can be used alone, or in combination with a pair of lateral recesses for instrumentation used during implantation. The anterior wall is substantially solid with a “flat” or fluidly continuous surface with coplanar access openings corresponding to a plurality (i.e., two and preferably three) of angled and preferably internally threaded passages. Advantageously three screws can be used wherein one project at one angle up or down and the other two are angled so as to splay in the opposite direction. This provides for a triangular base of fixation that is an advantage over two screws in helping to prevent rotation of the cage about the axis of the spinal column. This stability furthers an object of the invention to provide for relative stability of the adjacent vertebrae to facilitate spinal fusion.
The screw passages permit fixation means, and preferably screws to be inserted through the anterior wall of the cage through the end plates into the cancellous portion of adjacent upper and lower vertebrae to fix the vertebrae and the cage in position.
Further to hold the vertebrae and cage in position, the cage has a high friction surface which is illustrated in particular as a toothed, or ratchet surface to inhibit the cage from being pressed forward out of position. While this is the preferred surface, other contoured high friction surfaces could be used, having a combination of grooves or roughened or machined contours.
The cage is made from suitable strong and bio-compatible material such as titanium, surgical grade stainless, carbon composite, ceramic or the like.
As a separate aspect, the implant of the present invention (notably the substantially solid wall of a vertebral cage) has a unique locking screw assembly.
The issue of implant fixation using a bone screw has deserved and received significant attention by those skilled in the art. These issues are made more complex than more standard problems associated with the use of a screw to join two components by a number of factors. First, the screw and construct are not merely static, but become part of a dynamic system including the surrounding skeletal components as acted on by the soft tissues. Thus, the construct are often subject to large and quickly varied loads, including loads which might be applied during surgery to oppose strong muscular loads and to correct postural alignment. Thus, a skillful surgeon may actually use the stabilization construct to manipulate the components prior to fixing the construct to achieve the desired stabilization. Therefore, the anchoring mechanism is the skeleton which can be subject to significant stress. The best thread purchase can be achieved in the cortical layer of the bone, which is hard but brittle, and might therefore easily fracture. Cancellous bone allows a longer run for thread capture, but is softer, and more spongy in its consistency. In any case, while exact placement issues may be critical, the bone is relatively unforgiving to allow multiple attempts at fixation, particularly if the bone quality is compromised as it might be in cases that require surgical intervention. Second, the construct must be designed so that it is as benign as possible for implantation and afterwards. Components which may irritate or damage surrounding sensitive soft tissue are to be avoided. This is critical for anterior thoracic lumbar implants since this area is adjacent to important blood vessels. Thus, it is desirable to maintain fluid exterior implant shapes with low profiles, while maintaining sufficient material to ensure the structural integrity of the parts. It is critical to avoid shearing, splintering or breaking of the implants. Third, the complexities of the surgical context must be kept in mind. While the components are often relatively small they need to be easily assembled and “unfussy”. Thus, elegance in design is imperative. This is especially true since constructs may include numerous repetitive component parts or points of fixation. Thus, a time savings for a single sub-assembly may be multiplied for a number of such sub-assemblies. It is important that the parts go together easily, and can be disassembled if necessary. Problems such as cross-threading and incomplete assembly are to be avoided where possible. Fourth, the site of implantation may be difficult to access deep in the patient's body requiring the surgeon to utilize extended instruments through strong and thick muscular tissue to access the implantation site in an environment that causes limited visibility. Finally, the structure of bone must be kept in mind with respect to the issue of fixation.
In view of the foregoing concerns, one of the issues in the use of screws for implant fixation is inhibiting the screw from backing out of an implant recess. The implant may include the previously mentioned vertebral cage, or may also include any number of other implants including for example, plates, rod systems, holding flanges, and basically any internally threaded (and preferably through bore) implant component. Despite the broad range of applications for the self-locking screw assembly of the present invention, it is of particular advantage with the vertebral cage of the present invention since it allows for locking of the screw in the implant as well as for axial translation of the screw in the passage, and accordingly for play in the angulation of the screw, both movements being relative to the longitudinal axis of the screw and of the screw passage. This is very helpful to allow the screw to be fully screwed into the adjacent vertebral bodies so that the terminal surface or head of the screw will reside substantially flush with or below the smooth anterior surface of the cage defined by the access opening to the screw recesses. Thus, the screw can be tightened into a snug or locked engagement in the cage while drawing the cortical ring of the adjacent vertebral bodies unto the high friction surface of the cage, and the head can reside substantially, or fully within the screw recess to avoid the risk of contact with veins that run along the anterior portion of the spine.
The self-locking assembly comprises a specifically designed screw passage having female threads which are sized beyond the generally acceptable tolerances for the male threads of the fixation screws. Thus, while the pitch is about 0.1 inch, meaning the longitudinal distance needed to achieve one full rotation (360°) of thread spiral, is the same for the male and female locking threads. The profile of the locking thread of the male thread form is significantly smaller than the corresponding profile of the female locking thread form. The width of the thread profile for the male head is approximately two thirds of the corresponding thread profile in the female thread. The depth of thread engagement of the male thread form is approximately 90% of the depth of the corresponding female thread form. The cancellous thread on a first longitudinal screw area has a typical asymmetrical cross-section with a sharp spiral edge, while the locking thread on the second screw area (i.e., the head) has a symmetrical trapezoidal cross sectional shape which is about 60° on the front thrust surface or leading surface and the same on the back measured from the flat transverse plane of the spiral face which is less than 0.04 inch deep.
The thread of the first section (the cancellous section) forms a continuous, but interrupted spiral path with the thread of the second section (the locking section). Essentially, the male thread has a narrower foot print with a smaller major diameter) whereas the female thread has a narrower spiral ramp and a wider groove. This allows the screw to be translated along the longitudinal axis, which will change the relative radial position of the spiral thread at the point of entry in the bone. Also, there is play with respect to the angle of the screw within the passage. This design is forgiving with respect to the exact positioning of the screw relative to the passage and to the bone until the locking thread run out at the thrust face mates with the thrust face run out of the female thread and smoothly and securely locks the screw in position relative to the plate.